Supercritical fluid cleaning of banknotes and secure documents

ABSTRACT

A method and apparatus for cleaning a stack of secure instruments such as banknotes is disclosed. Each secure instrument or banknote includes a substrate, visual data and a security feature. The apparatus includes a chamber configured to contain a supercritical fluid at a temperature and a pressure and for a duration sufficient to disinfect or decontaminate the banknotes and not compromise the security feature and the visual data of the banknotes, a structure for holding the stack of banknotes in the chamber so that the supercritical fluid circulates through and around the banknotes to remove one or more substances into the supercritical fluid, and a strapping machine configured to secure a securing mechanism to the stack of banknotes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalApplication No. 61/977,560, filed Apr. 9, 2014. This application is alsoa continuation-in-part of U.S. patent application Ser. No. 14/246,882,filed Apr. 7, 2014, which is a continuation-in-part of U.S. patentapplication Ser. No. 14/021,603, filed Sep. 9, 2013, which claims thebenefit of priority of U.S. Provisional Application Ser. No. 61/721,296,filed Nov. 1, 2012, the entire contents of each of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to the cleaning of securedocuments such as banknotes without inducing damage thereof. Morespecifically, the present invention relates to the use of supercriticalfluids to clean secure documents or banknotes without damaging theirvisual data, inks, substrates or security features. The process is alsoeffective in disinfecting the secure documents or banknotes.

BACKGROUND OF THE INVENTION

High security documents such as banknotes have substrates formed fromvarious materials. In the United States, paper currency is made from anon-woven combination of 75% cotton and 25% linen fibers. In most othercountries, pulp-based substrates are used. Some countries, such asCanada, have used cotton and paper blended banknotes. In addition,countries such as Australia, New Zealand and Canada have issuedbanknotes having polymer substrates, e.g., substrates includingbiaxially oriented polypropylene. The substrate, which may include oneor more plies of the substrate material, may include security featuressuch as laminated polymer or paper security threads, planchettes, andwatermarks formed directly into the substrate. For example, U.S. papercurrency contains small segments of red and blue fibers scatteredthroughout for visual identification.

Banknotes also include visual data printed on the substrates. The visualdata may include images such as portraits, authentication informationsuch as serial numbers, or both. The inks used to print on thesubstrates may include special dry color pigments blended with oils andextenders and phosphor chips containing layered micro-interferencelayers. Such inks include Flexo inks, gravure inks, and thicker intaglioinks.

High security documents such as banknotes are generally formed onsubstrate materials that are frequently equipped with security elements,which are difficult to imitate and which permit even a layman to checkthe authenticity of the printed information or the document. Securityelements can be, for example, windowed security threads, which arevisible in certain areas on the surface of the banknote, applied foils,which have a transparent or metallized embossed hologram, blindembossings, so-called “latent images” produced by printing technology orby printing and embossing technology, which render different informationfrom different viewing angles, prints containing optically variablepigments and producing different color effects depending on the viewingangles, and prints comprising metallic effect ink, which have metallicluster, for example, in a gold, silver or bronze tone. In addition tothese unaided features, there are quasi-public security threads, fibersand inks, which fluoresce or phosphoresce under illumination withultraviolet (“UV”) or infrared (“IR”) sources.

Other security features in paper currency include numeric watermarks,Guilloche patterns, which are narrow geometric patterns created by ageometric lathe or mathematically, microprinting, digital watermarks,magnetic inks and threads, demetalized security threads, holographicfeatures, fluorescent inks, lenticular lens array security threads, andfluorescent and non-fluorescent security threads.

High level covert security features include ENIGMA (De La RueInternational) and M (Geiseke and Devrient). An important securityfeature in currency is the M feature, where “M” refers to “machinereadable.” The M feature is a colorless, inorganic oxide integrated intothe paper substrate, the printing ink, security ink, or a securitythread, without causing any change in the appearance of the banknote.The powdered M feature may be blown into the paper substrate in a trailto identify a particular banknote denomination. When exposed to a flashfrom a strong source of light, the M feature emits a band of light in asplit second that rapidly disappears. This repeatable, characteristiclight band of the banknote can be authenticated by a reading device. Thecentral banks protect the security of the M feature by requiring the useof special sensors to recognize it.

As counterfeiters have become more sophisticated, the security featuresin such documents have had to become more advanced as well in order toprevent widespread fraud. As the substrates of such secure documentshave become more advanced, the cost to produce them has also increased,thus making the replacement of worn currency quite expensive. Therefore,it is important that in addition to being secure, such documents musthave a high level of durability.

Banknotes are removed from circulation for a variety of reasons. Basedon one study, 81% of notes are removed because of soiling, 9% areremoved because of damage caused by mechanical means, especiallytearing, 5% are removed because of graffiti on the notes, 4% are removedbecause of general wear and tear, and 1% are removed because of damageto the security elements. Generally, 60% to 80% of all rejected banknotes result from to soiling.

Banknotes have a finite time in circulation due to soling and tearing ofthe notes in use by the public. For example, it takes about 4,000 doublefolds (first forward and then backward) before a U.S. paper bill willtear. Banknotes are handled in many ways during their usable life andexperience a variety of mechanical stresses, as well as being broughtinto contact with substances that can dirty the notes, resulting indifficulty in their authentication and use. One of the majordeterminants of the banknote life, which is shortest for the lowestdenominations, is soiling. Work by the Dutch National Bank has shownthat the primary source of soiling is deposited sebum following contactwith fingers, which sebum eventually oxidizes and becomes yellow.Further, a study by the Microbiology Department of Karachi University inPakistan concluded that currency notes could also carry contaminantsthat cause diarrhea and urinary tract infections, in addition to skinburning and septicaemic infection. One study found that 26% of notescontained high levels of bacteria, and 80% of notes had some traces ofbacteria. An even more concerning finding was that pathogens, includingbacteria and viruses, on banknotes have the potential to developresistance to antibiotics, making the treatment of infectious diseasesmore difficult.

Such “dirty” money is not simply confined to developing nations. Some ofthe studies on contaminated currency emerging from the United Stateswere equally revealing. In a recent survey conducted for the Departmentof Endocrinology at the Wright-Patterson Medical Center in Ohio,researchers collected 68 one-dollar notes from a concession stand at ahigh school sporting event and a grocery store check-out counter, andexamined them for bacterial contamination. Only four bills (six percent)contained no detectable germs.

Given the huge amounts of banknotes in circulation for even smallcountries, determining the fitness of banknotes is not only ofimportance in cost control, but also poses a serious technical challengein terms of processing speed and accuracy. Moreover, the extent ofdirtiness of a banknote cannot easily be captured in objective rules. Asa result, not only is accurate determination of the fitness of banknotesof interest from a cost point of view, but also cleaner notes are moresecure and more attractive to the public. Studies have shown thatsoiling is one of the primary reasons for classifying banknotes unfitfor circulation by banknote fitness sensors using both white light andspecific wavelength sources.

In order to improve durability and soil resistance of these substrates,it is known to use documents of value with a dirt-repellent and/ormoisture resistant protective layer to extend the documents' lifetimeand fitness for circulation. Such a protective layer typically containscellulose ester or cellulose ether for the greater part and micronizedwax for a lesser part, and is applied all over the banknotes. Themicronized wax is dispersed by kneading or mixing with oil, an inkbinder or a mixture thereof. The sheets freshly printed with theprotective layer can be stacked without difficulties and without anyblack ink from one sheet staining the sheet below.

Another coating composition containing only a binder and no fillers hasbeen applied to the banknote paper, which has a large surface area orhigh surface roughness due to its porosity. The composition is appliedin a layer and has a thickness with a smooth surface, thus having littlepossibility for resulting dirt deposits. Further, the coating is thinenough not to impair the other stated properties of the paper.

A problem with this approach is that known protective layers do not lastor wear well. Conventional protective layers comprising water-basedlacquers usually fail to completely meet a demanding requirementprofile. For example, very good dirt repellence and adhesion qualitiescontravene resistance to the penetration of liquid, and vice versa.Water-based lacquers, therefore, currently meet the high requirementsfor a protective layer in security printing, and in particular banknoteprinting, only if a second component in the form of a crosslinking agentis added.

Another problem relating to banknotes is that central banks need toreplace worn and soiled notes at a cost to taxpayers. In the UnitedStates, the volume of notes manufactured is in the billions of notes peryear (4-6 billion typically). The production of banknotes is costly,particularly so for the higher denominations, which have many securityfeatures that are both accessible to the public and machine readable bybill acceptors and the central banks using high speed sorters. Banknotesorters made by Geiseke and Devrient, De La Rue International andToshiba typically process banknotes at rates of 10-40 banknotes/secondand perform a number of diagnostics using sensors in the notes' travelpath. These sensors are a combination of authentication sensors as wellas note fitness sensors. The fitness sensors primarily use imaging andanalysis of the captured images to determine if the banknote should bedestroyed or returned to circulation.

The cost of replacing banknotes is significant as the higherdenominations contain Level I, II and III security features for use bythe public, commercial banks, single note acceptor devices and centralbanks. In the United States, for example, the currency replacementbudget is $747 million and breaks down as follows:

-   -   $1 and $2 notes—5.2 cents per note    -   $5 and $10 notes—8.5 cents per note    -   $20 and $50 notes—9.2 cents per note    -   $100 note—7.7 cents per note    -   $100 note to be released in October 2013—13 cents per note

With over 150 billion new banknotes being manufactured and printed everyyear around the world, the cost of replacement of unfit currency hasapproached $10 billion annually. In addition to the replacing the notes,there is a sizable waste disposal cost associated with the destructionof the shredded notes that are determined to be unfit. This amounts toabout 150,000 tons of waste worldwide annually, based on total worldwidecirculation of 150 billion notes. This is particularly problematic forpolymer notes, which also pose larger environmental problems withrespect to burning and landfill disposal.

Based on these facts, there is a need to employ a manner for cleaningbanknotes, which are soiled but not torn or ripped, that does not attackthe print and security features of the note. There is still a furtherneed for a system that applies a certain class of fitness parameters tocause identified banknotes to be cleaned using a method that does notattack the print and security features before making a determinationthat they should either be returned to circulation or destroyed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and systemto clean banknotes that are soiled but not torn or ripped, which methoddoes not attack the print and security features of the banknotes. It isa further object of the present invention to provide a system thatapplies a certain class of fitness parameters to cause identifiedbanknotes to be cleaned using a supercritical fluid that does not attackthe print and security features of the banknotes before making adetermination that they should either be returned to circulation ordestroyed.

In general, in one aspect, the invention features an apparatus forcleaning a stack of banknotes, each banknote including a substrate,visual data and a security feature. A chamber is configured to contain asupercritical fluid at a temperature and a pressure and for a durationsufficient to disinfect or decontaminate the banknotes and notcompromise the security feature and the visual data of the banknotes. Astructure holds the stack of banknotes in the chamber so that thesupercritical fluid circulates through and around the banknotes toremove one or more substances into the supercritical fluid. A strappingmachine is configured to secure a securing mechanism to the stack ofbanknotes.

Implementations of the invention may include one or more of thefollowing features. The apparatus may include a sorter for determiningwhether the secure instruments have one or more properties that satisfyone or more predetermined criteria. The sorter may be configured tooperate with the strapping machine. The securing mechanism may include aband, a strap or a clip. The supercritical fluid in the chamber may becombined with an additive for removing marks on the banknotes, and theadditive may include an oxalic acid. The supercritical fluid in thechamber may be combined with an additive for enhancing at least onemechanical property of the banknotes, and the additive may include atleast one of an aqueous citric acid solution and ammonium zirconiumcarbonate. The structure may be a basket. The stack of banknotes mayinclude a material covering the stack of banknotes and having at leastone slit allowing the supercritical fluid to circulate through andaround the banknotes. To disinfect or decontaminate may include toremove dangerous chemicals from the banknotes, and the dangerouschemicals may include pesticides or mercury.

In general, in another aspect, the invention features an apparatus forcleaning a stack of banknotes, each banknote including a substrate,visual data and a security feature. A chamber contains a supercriticalfluid at a temperature and a pressure and for a duration sufficient toclean the secure instruments and not compromise the security feature andthe visual data of the banknotes. A structure holds the stack ofbanknotes in the chamber so that the supercritical fluid circulatesthrough and around the banknotes to remove one or more substances intothe supercritical fluid.

Implementations of the invention may include one or more of thefollowing features. The supercritical fluid may contain at least oneadditive. The additive may be for removing marks on the secureinstruments, and the additive may include an oxalic acid. The additivemay be for enhancing at least one mechanical property of the secureinstruments, and the additive may include at least one of an aqueouscitric acid solution and ammonium zirconium carbonate. To clean thebanknotes may include removing pathogens from the banknotes, and thepathogens may include bacteria and viruses.

In general, in another aspect, the invention features a method forcleaning at least one stack of secure instruments, each secureinstrument including a substrate, visual data and a security feature.The method may include exposing the stack to a supercritical fluid at atemperature and a pressure and for a duration sufficient to clean eachsecure instrument and not compromise the security feature and the visualdata of each secure instrument, and maintaining a securing mechanism onthe stack during exposure of the stack to the supercritical fluid, suchthat clean each secure instrument may include removing one or moresubstances from each secure instrument into the supercritical fluid.

Implementations of the invention may include one or more of thefollowing features. The method may include securing the securingmechanism to the stack and using a strapping machine to secure thesecuring mechanism to the stack. The securing mechanism may be a strap,a band, or a clip. Exposing a second stack of secure instruments to thesupercritical fluid. Exposing the stack to the supercritical fluid mayinclude flowing the supercritical fluid through and around the secureinstrument in a chamber. The method may include sorting the stack ofsecure instruments based on one or more criteria. The supercriticalfluid may be combined with an additive for removing marks on the secureinstrument or for enhancing at least one mechanical property of thesecure instrument. The additive may include at least one of oxalic acid,an aqueous citric acid solution, and ammonium zirconium carbonate. Themethod may include placing the stack in a device in a supercriticalfluid cleaning chamber prior to exposing the stack to the supercriticalfluid. The device may be a basket, and the method may includetransitioning the device between a position in the supercritical fluidcleaning chamber and a position outside of the supercritical fluidcleaning chamber.

In general, in another aspect, the invention features an apparatus forcleaning a stack of secure instruments, each secure instrument includinga substrate, visual data and a security feature. The apparatus mayinclude a chamber containing a supercritical fluid at a temperature anda pressure and for a duration sufficient to clean the secure instrumentsand not compromise the security feature and the visual data of thesecure instruments and a strapping machine configured to secure asecuring mechanism to the stack of secure instruments.

Implementations of the invention may include one or more of thefollowing features. The apparatus may include a structure for holdingthe secure instrument in the chamber so that the supercritical fluidcirculates through and around the secure instrument to remove one ormore substances into the supercritical fluid. The apparatus may includea sorter for determining whether the secure instruments have one or moreproperties that satisfy one or more predetermined criteria, and thesorter is configured to cooperate with the strapping machine. Thesecuring mechanism may be a band, a strap, or a clip. The supercriticalfluid in the chamber may be combined with an additive for removing markson the secure instruments or for enhancing at least one mechanicalproperty of the secure instruments, and the additive may include atleast one of oxalic acid, an aqueous citric acid solution, and ammoniumzirconium carbonate. The apparatus may include a device disposed in thechamber for maintaining the secure instruments in a desired position,and the device may be a basket.

In general, in another aspect, the invention features a method forcleaning a secure instrument including a substrate, visual data and asecurity feature. The method may include exposing the secure instrumentto a supercritical fluid combined with at least one additive at atemperature and a pressure and for a duration sufficient to clean thesubstrate and not comprise the security feature and the visual data,such that to clean the substrate may include removing one or moresubstances from the substrate into the supercritical fluid.

Implementations of the invention may include one or more of thefollowing features. The additive may be for removing marks on the secureinstruments or for enhancing at least one mechanical property of thesecure instruments, and the additive may include at least one of oxalicacid, an aqueous citric acid solution, and ammonium zirconium carbonate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other aspects, features and advantages can bemore readily understood from the following detailed description withreference to the accompanying drawings, wherein:

FIG. 1 is a supercritical fluid phase diagram for carbon dioxide;

FIG. 2 is a flow chart showing the cleaning and sorting of banknotes inaccordance with one embodiment of the present invention;

FIG. 3 is a flow chart showing the cleaning and sorting of banknotes inaccordance with another embodiment of the present invention;

FIG. 4 is a flow chart showing the cleaning cycle of the presentinvention;

FIG. 5 is an exemplary high pressure supercritical fluid chamber;

FIG. 6 is a comparison of the same part of a U.S. $1 banknote before andafter coating and oxidation with a sebum layer;

FIG. 7 shows the spectra for an uncirculated U.S. $1 banknote, the U.S.$1 banknote after the application of sebum, and the U.S. $1 banknoteafter supercritical fluid cleaning.

FIGS. 8A and 8B show the reflectance spectra of banknotes coated withoxidized sebum both before and after supercritical CO₂ cleaning inaccordance with the present invention;

FIG. 9 shows the results of cleaning a 5 Euro note in accordance withthe present invention;

FIG. 10 shows the diffuse reflection spectra of the bank note of FIG.10A before and after cleaning with the supercritical fluid;

FIG. 11 shows the fluorescence spectra of security threads in banknotesboth before and after cleaning in accordance with the present invention;

FIGS. 12A and 12B show the fluorescence spectra of security threads inbanknotes both before and after cleaning in accordance with the presentinvention;

FIG. 13 shows the robustness of the UV excited emissive features insecurity fibers of a Russian Ruble;

FIG. 14 shows the robustness of the UV excited emissive securityfeatures in printing on a Chinese Yuan;

FIG. 15 shows the robustness of the UV excited emissive securityfeatures in printing on a British Pound before and after exposure to asupercritical fluid;

FIGS. 16A and 16B show the transient responses for a U.S. $1 note whichis in circulation and which is not in circulation, respectively;

FIGS. 17A and 17B show the signals of the uncirculated note of FIG. 16Bbefore and after cleaning with a supercritical fluid;

FIG. 18 shows a stack of U.S. $1 banknotes bound together with a strap;

FIG. 19 is an exemplary supercritical fluid cleaning chamber accordingto an embodiment of the invention; and

FIGS. 20A and 20B is an exemplary supercritical fluid cleaning chamberaccording to another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides for the cleaning of secure documents suchas banknotes using supercritical fluids. More specifically, the presentinvention provides a method of cleaning secure documents and banknotesusing supercritical fluids in a manner that does not damage or otherwisecompromise their visual data, inks, substrates or the security featurescontained therein. The security features and visual data are notcompromised if they remain recognizeable to the public, or upon machinereadable examination, for their intended purpose. The substances thatcan be removed from the substrates of secure documents includecontaminants, dirt, sebum from users' hands, and pathogens includingbacteria and viruses.

Such cleaning may also have the effect of disinfecting the banknotes.For example, in some instances, it may be desired to disinfect ordecontaminate the banknotes from dangerous chemicals, including but notlimited to pesticides, mercury, and the like. Banknotes containing thesedangerous chemicals may need to be cleaned for hygienic and/or safetypurposes, enabling individuals to handle the banknotes to handle thebanknotes to determine, for example, if the banknotes are counterfeits.

In some instances, chemicals on the banknotes may result in banknotesthat are unsafe for handling by humans prior to disinfection ordecontamination. Accordingly, in one embodiment, supercritical fluidcleaning of the banknotes may be achieved when the banknotes are coveredby a material that prevents human contact with the banknotes. Thematerial covering the banknotes may be any material known to thoseskilled in the art that may be configured to withstand desiredtemperature and pressure values within a supercritical fluid cleaningchamber. For example, in one embodiment, the banknotes may be covered inshrink wrap during cleaning. The material covering the banknotes mayinclude at least one opening therein. The opening may be any size,shape, or configuration such that it may be small enough to preventhuman contact with the banknotes, but large enough to enablesupercritical fluid to flow there through and around the banknotes fordisinfection. In one embodiment, for example, the at least one openingmay include a plurality of slits.

It is estimated that the use of supercritical fluid cleaning will allowfor a 10% reduction in the number of banknotes that are replacedannually, while allowing a significant percentage of soiled banknotes tobe returned to circulation, thus saving governments worldwideapproximately $1 billion annually and reducing the environment impactassociated with unfit banknotes. At a 10% reduction in banknote annualproduction, the estimated decrease in the carbon footprint is 10⁶ tonsof equivalent CO₂.

FIGS. 2 and 3 illustrate systems for cleaning banknotes and securedocuments according to embodiments of the present invention. Asillustrated in FIGS. 2 and 3, the systems may be configured to cooperatewith at least one supercritical fluid for cleaning banknotes and securedocuments. Generally, supercritical fluids mixed with other gases andadditives including ionic liquids, are effective solvents for a varietyof organics and have been used in a number of cleaning and extractionapplications including pharmaceutical manufacturing, perfume production,and decaffeination. Accordingly, in the embodiments of the presentinvention, any supercritical fluid known to those skilled in the art maybe employed, so long as the supercritical fluid may be configured to aidin cleaning banknotes and secure documents.

Examples of supercritical fluids that may be used alone or incombinations thereof include, but are not limited to, CO₂, N₂O, CO, andSF₆. In particular, CO₂ may be a supercritical fluid used alone or incombination with trace amounts of other supercritical fluids, including,but not limited to, N₂O, CO, and SF₆. For example, N₂O creates a degreeof solubility in the system that cannot be accomplished with CO₂ alone,and SF₆ may be particularly useful in a cleaning system because of itshighly electronegative properties.

FIG. 1 illustrates the supercritical fluid phase diagram for carbondioxide. As illustrated in FIG. 1, CO₂ has a supercritical point at 72.9atm and 304.25 K. When in the supercritical phase, the CO₂ material hasa density approaching that of the liquid but has the space fillingproperties of a gas-like substance. When exposed to CO₂ in asupercritical state, many organic materials become soluble withoutchemical attack in certain regions of the phase diagram. In particular,the materials may be removed into the supercritical fluid when theirfree energy is lowered. In particular, oily substances such as sebum(including after oxidation or hydrolysis), which is a major contributorto banknote soiling, as well as other oils and contaminants, are solublein supercritical CO₂ and other supercritical fluid mixtures. Animportant point to note is that the banknotes, after this cleaning, aredry since CO₂ sublimates at room temperature and pressure. In addition,CO₂ as a supercritical cleaning agent has very low environmental impactas one of the lowest impact greenhouse gas components. Any environmentalimpact associated with the use of CO₂ is minimal compared to the costand negative environmental impact of disposing of unfit currency, e.g.,by burning or in landfills. Further, CO₂ can be recycled for reuse andrecirculation in the cleaning system after filtering out contaminants.

Additives may be combined with supercritical fluids to enhancecleanliness and other properties of banknotes and secure documentsduring the cleaning process. The additives may include, but are notlimited to, mixtures of oxalic acid and water, methanol and/or ethanol,aqueous citric acid solutions, ammonium zirconium carbonate (AZC), andcombinations thereof. In addition, the amount of additive(s) combinedwith the supercritical fluid(s) may be any desired amount known to thoseskilled in the art so long as the amount of additive(s) enables adesired result to be achieved. In one embodiment, for example, theadditive(s) may be approximately 10% by volume to the fluid phase of thesupercritical fluid.

The additive(s) combined with the supercritical fluid(s) during thebanknote cleaning process may be chosen based on the state of thebanknotes prior to cleaning and/or the desired state of the banknotesafter cleaning. For example, if the banknotes to be cleaned includemarks from writing implements (e.g., pens or markers), an additive maybe chosen that includes a mixture of oxalic acid and water and/ormethanol. Particularly, experiments have shown that saturated solutionsof oxalic acid in water may be effective for removing gel pen markings;saturated solutions of oxalic acid in methanol or ethanol may beeffective for removing permanent marker markings; and mixtures of oxalicacid with water/methanol may be effective for removing ball point penmarkings, as well as gel pen markings and permanent marker markings.

Additives may also be chosen to strengthen the banknotes duringcleaning. Generally, mechanical wear from handling and folding ofbanknotes may cause a network of cellulosic fibers on the banknotes tobecome porous, which may result in mechanically limp banknotes.Accordingly, at least one additive may be combined with thesupercritical fluid(s) that may lead to cross-linking of cellulose,which may result in at least a partial reversal of banknote limpness.For example, in one embodiment, use of aqueous citric acid solutionswith a supercritical fluid may result in cross-linking of cellulosicfibers on the banknotes by self-catalyzed esterification of cellulosichydroxyl groups. In addition, or alternatively, ammonium zirconiumcarbonate (ACZ) may be used as an additive, since ACZ may be configuredto cross-link cellulosic fiber and also may be configured to act as acoupling agent to graft starch onto the cellulosic fibers (generally,grafted starch on fiber surfaces may improve the bonding capabilities ofthe fibers).

FIGS. 2 and 3 illustrate systems for cleaning banknotes according toembodiments of the present invention. As illustrated in each of thefigures, the systems may each include a sorting machine, such as thoseused by central banks. Central banks use high speed sorting machines,which are fitted with optical and mechanical inspection systems thatinvestigate the banknotes to determine if they must be destroyed or canbe sent back into circulation. In particular, such high speed sortingmachines can be used to interrogate banknotes for both authenticity andfitness. The largest sorting machines operate at 40 banknotes per secondand can have as many as 16 sensors to remove counterfeits and notes thatare not fit for recirculation. Accordingly, the sorting machines of thepresent invention may include fitness sensors that may be configured tooperate primarily on optical image analysis and examine a number ofparameters including tears, tapes, graffiti and soiling. Other sensorsmay be used to determine banknote limpness as another metric fordetermining when the notes are fit or have to be replaced. In addition,banknotes may be authenticated to determine whether or not they arecounterfeit using the notes' security features, including both publicand machine readable security features. Authentication information,which may be machine readable, may also be alphanumeric or image dataprinted on the banknotes.

The fitness sensors may be configured to analyze incoming banknotes andselect notes which are unfit due to soiling but are otherwise stillviable in terms of limpness and lack of tears, rips and graffiti. Theseparameters for acceptable fitness characteristics can be determined andoptimized by the specific central bank based on population trends andbanknote acceptance metrics. In one embodiment, shown in FIG. 2, allbanknotes may be cleaned in a supercritical fluid cleaning chamber andthen sorted for either recirculation or destruction and/or shredding,depending on whether they meet the predetermined fitness criteria. Asshown in FIG. 3, in another embodiment, banknotes that are fit but fortheir soil level can be routed to a supercritical fluid cleaningchamber.

Banknotes that are selected for cleaning may be placed in a chamber(e.g., FIG. 5), where supercritical CO₂ may be applied at an optimalpressure, temperature, and duration for the specific banknotedenominations, designs and substrates to remove the soil deposits fromthe banknote. The required or optimal temperature, pressure and durationwill depend on the liquid or liquids in the supercritical fluid, as wellas any additions such as ionic fluids or other gases. FIGS. 2 and 3illustrate that the cleaned banknotes may then be routed to a secondsorting system, which accepts the supercritical fluid-cleaned notes andperforms a fitness measurement to qualify those ready for reuse by thepublic from those that were not successfully cleaned. The latter may besorted and separated for destruction and/or shredding.

The banknotes may be cleaned in the chamber individually or in stacks ofmultiple banknotes. Each stack of banknotes may include any number ofbanknotes known to those skilled in the art so long as each banknote inthe stack is capable of being cleaned within the chamber. In oneembodiment, for example, each stack of banknotes may includeapproximately 100 banknotes. The stacks of banknotes may be cleaned inthe chamber individually or in bundles (i.e., multiple stacks at onetime). For example, the chamber may be configured to clean a bundle thatincludes at least 5 stacks of banknotes.

Each stack of banknotes may include a securing mechanism. The securingmechanism may be any mechanism known to those skilled in the art thatmay be configured to maintain a desired number of banknotes together ina stack. The securing mechanism may further be any desired material,shape, size and/or configuration so long as the securing mechanismenables the stack of banknotes to be cleaned (either by removal of thesecuring mechanism or while the securing mechanism secures the stack ofbanknotes). For example, the securing mechanism may include, but is notlimited to, a flexible, rigid, or elastic band, a strap, or a clip. FIG.18 illustrates an example of a flexible band, which may be wrappedaround a stack of banknotes and held together via any closure mechanismknown to those skilled in the art, including, but not limited toadhesives.

In some embodiments the stacks of banknotes may be cleaned in thechamber with the securing mechanism thereon. Alternatively, in otherembodiments, the securing mechanism may be removed prior to cleaningeach stack. Accordingly, the embodiments of FIGS. 2 and 3 may beconfigured to cooperate with a banknote strapping machine (not shown)that may be configured to secure a stack of banknotes with a securingmechanism and/or remove a securing mechanism from a stack of banknotes.

The strapping machine may be any strapping machine known to thoseskilled in the art that may be configured to perform desired functions,including but not limited to, removing a securing mechanism from a stackof banknotes, securing a securing mechanism on a stack of banknotesand/or transitioning a stack of banknotes between the strapping machineand a system for cleaning banknotes. Additionally, the strapping machinemay be configured to perform the desired functions without damagingfeatures of the banknotes. For example, the strapping machine may be acommercially available product, such as an Easy-Banker money binder.

The strapping machine may include a removal mechanism for removing thesecuring mechanism from the stack of banknotes. The removal mechanismmay include any securing mechanism removal features known to thoseskilled in the art, including, but not limited to, features that enablebreaking, cutting, opening and/or pulling apart of the securingmechanism. In addition, or alternatively, in an embodiment where thesecuring mechanism is elastic, the removal mechanism may be configuredto expand the securing mechanism and remove it from the stack ofbanknotes.

In addition, or alternatively, in one embodiment, the strapping machinemay include a binding mechanism for securing a stack of banknotes withthe securing mechanism. The binding mechanism may include any featuresknown to those skilled in the art and configured for securing a stack ofbanknotes together with the securing mechanism. The features of thebinding mechanism may include, but are not limited to, wrapping, tyingand/or clasping. In addition, or alternatively, in one embodiment, afeature of the binding mechanism may enable expansion and placement ofan expandable securing mechanism on the stack of banknotes.

As previously discussed, the strapping machine may be configured tocooperate with systems for cleaning banknotes, such as those depicted inthe embodiments of FIGS. 2 and 3. For example, in the embodiments ofFIGS. 2 and 3, the strapping machine may be configured to transfer astack of banknotes to the supercritical fluid chamber where thebanknotes may be cleaned. Alternatively, in the embodiment of FIG. 3,the strapping machine may be configured to transfer the stack ofbanknotes to the sorting machine.

In embodiments where stacks of banknotes may be cleaned with a securingmechanism thereon, the strapping machine may be configured to cooperatewith the systems such that entire stacks of banknotes may betransitioned therebetween. Alternatively, or in addition, in embodimentswhere the securing mechanism is not on a stack of banknotes duringcleaning, the strapping machine and the systems may be configured tocooperate such that at least one banknote at a time may be transitionedtherebetween. For example, in one embodiment the banknotes may betransitioned individually. Alternatively, or in addition, the strappingmachine may be configured to transition multiple banknotes at a time.

Banknotes that have been sorted or that are otherwise being subjected tothe cleaning process of the present invention may be placed thesupercritical fluid cleaning chamber. FIG. 19 illustrates an exemplarysupercritical fluid cleaning chamber 10 according to an embodiment ofthe invention. As illustrated in FIG. 19, the supercritical fluidcleaning chamber 10 may include a chamber structure 12. Chamberstructure 12 may be any structure know to those skilled in the art thatmay be configured to hold at least one banknote in a position thatenables supercritical fluid to flow around the banknote.

Chamber structure 12 may include a holding structure 14. The holdingstructure 14 may be any suitable size, shape, or configuration, and mayinclude an interior portion configured to hold at least one banknotetherein. For example, as illustrated in FIG. 19, the interior portion ofthe holding structure 14 may be configured to hold multiple stacks ofbanknotes at one time for cleaning. The holding structure 14 may be inthe form of a capsule having rounded first and second ends 14 a, 14 b.Alternatively, the holding structure may be square, rectangular,circular, cylindrical, or any other desired shape. At least one of thefirst and second ends 14 a, 14 b of the holding structure 14 may beconfigured to transition between an open configuration (e.g., second end14 b in FIG. 19) and a closed configuration (e.g., first end 14 a inFIG. 19). When the first and second ends 14 a, 14 b are in the closedconfiguration, the interior of the holding structure 14 may beconfigured to reach a desired temperature and pressure for supercriticalfluid cleaning of banknotes therein.

FIG. 19 illustrates that the holding structure 14 may be seated on abase portion 16 of the chamber structure 12. The holding structure 14may be removably or permanently attached to the base portion 16. In someembodiments, the base portion 16 may be fixed to a surface.Alternatively, as illustrated in FIG. 19, the base portion 16 mayinclude wheels, or any other suitable device known to those skilled inthe art that may be configured to enable the chamber structure 12 totransition across a surface between multiple locations.

Banknotes placed in the holding structure 14 may be positioned such thata supercritical fluid may flow around the banknotes. In some embodimentsthe banknotes may be positioned individually into the holding structure14. Alternatively, the banknotes may be positioned in the holdingstructure 14 in stacks. The banknotes may also be positioned to enableease of their placement in and removal from the holding structure 14.For example, in embodiments where the holding structure 14 has arelatively shallow depth (e.g., a device operator can reach an interiorside of the first end 14 a of the holding structure), the banknotes maybe positioned directly on an interior surface of the holding structure14.

Alternatively, or in addition, the holding structure 14 may include adevice therein for maintaining the banknotes in a desired position. Forexample, the holding structure 14 may include shelves for placement ofthe banknotes. Alternatively, or in addition, as illustrated in FIG. 19,the device for maintaining the banknotes in a desired position may be abasket 18. The basket 18 may any size, shape, or configuration such thatit may be configured to fit within the holding structure 14 when thefirst and second ends 14 a, 14 b are in the closed configuration andsuch that it may be configured to enable supercritical fluid to flowaround the banknotes therein. The basket 18 may additionally be of anydesired material known to those skilled in the art and configured tomaintain its structure under the required temperature and pressureconditions for supercritical fluid cleaning of the banknotes. Forexample, as illustrated in FIG. 19, the basket 18 may be a mesh.

FIG. 19 additionally illustrates that the basket 18 may include aninterior portion 20 configured to hold at least one stack of banknotes.The banknotes may be stacked on top of each other and/or side by side.In one embodiment, the interior portion 20 of the basket 18 may includea single compartment. Alternatively, as illustrated in FIG. 19, theinterior portion 20 of the basket 18 may include multiple compartments24. The basket 18 may include any desired number of compartments 24known to those skilled in the art so long as each compartment 24 may besized and shaped to maintain at least one stack of banknotes therein andso long as supercritical fluid may be configured to flow throughout thebasket 18 and around the banknotes.

The basket 18 may be configured to transition between an openconfiguration, enabling at least one stack of banknotes to be placed inor removed from the interior portion 20 of the basket 18, and a closedconfiguration, enabling supercritical fluid cleaning of the banknoteswithin the holding structure 14. For example, at least one wall of thebasket 18 may be movable relative to an interior of the basket 18. FIG.19 illustrates that a top wall 22 of the basket 18 may be movablerelative to the interior portion 20 of the basket. Alternatively, or inaddition, as illustrated in FIG. 19, a side wall 26 of the basket 18 maybe movable relative to the interior portion 20. FIG. 19 illustrates thateach compartment 24 in the basket 18 may include a corresponding sidewall 26. In alternative embodiments, a single side wall may correspondto at least two compartments.

The basket 18 may be configured to transition in and out of the holdingstructure 14 to facilitate loading and unloading of stacks of banknotestherein. In some embodiments, the basket 18 may be separate from theholding structure 14. Alternatively, as illustrated in FIG. 19, thebasket 20 may be integral with the holding structure 14 such that thebasket 18 and the holding structure 14 may be configured to remain incontact upon removal of the basket 18 from the holding structure 14.FIG. 19 illustrates that the basket 18 may be able to transition in andout of the holding structure 14 via at least one rail 28. For example,the basket 18 may include at least one rail 28 attached thereto along abottom wall and/or a side wall. Additionally, the interior of theholding structure 14 may include at least one corresponding rail (notshown) along which the at least one rail 28 on the basket 18 may slide.There may also be at least one stop located on the at least one rail 28and configured to maintain the at least one rail 28 within thecorresponding interior rail, preventing the basket 18 from separatingfrom the holding structure 14.

FIGS. 21A and 21B illustrate an exemplary supercritical fluid cleaningchamber 100 according to another embodiment of the invention. Theembodiment of FIGS. 21A and 21B may include many of the same features asdiscussed in the embodiment of FIG. 19. For example, the supercriticalfluid chamber 100 of may include a chamber structure 112, which mayinclude a holding structure 114 having a first end 114 a and a secondend 114 b and a base portion 116. At least one of the first and secondends 114 a, 114 b may be configured to transition between an openconfiguration and a closed configuration. The supercritical fluidchamber 100 of FIGS. 21A and 21B may further include a basket 118configured to maintain stacks of banknotes in a desired position withinthe holding structure 114.

In the embodiment of FIGS. 21A and 21B, the basket 118 may not beintegral with the holding structure. That is, the basket 118 may beconfigured to transition between a position within the holding structure114 (FIG. 20B) and a position outside of the holding structure 114 (FIG.20A) in which the basket 118 is no longer in contact with the holdingstructure 114. Accordingly, the supercritical fluid chamber 100 mayinclude any mechanisms known to those skilled in the art configured toaid in this transitioning.

For example, as illustrated in FIGS. 21A and 21B, a cart 117 may be usedto facilitate transitioning of the basket 118 in and out of the holdingstructure 114. The cart 117 may be sized, shaped, and configured tosupport the basket 118 with stacks of banknotes therein and may bemovable relative to the chamber structure 112. The cart 117 may includea height that may be substantially the same as the height of the baseportion 116 of the chamber structure 112 so that a device user does nothave to vertically move the basket 118 as it is transitioned in and outof the holding structure 114. Additionally, the cart 117 and an interiorportion 129 of the holding structure may each include at least one rail128. A corresponding rail (not shown) may be located on a bottom surfaceof the basket 118 such that the basket 118 may be configured to slidealong each of the interior surface 129 of the holding structure and atop surface of the cart 117 as the basket 118 is transitioned in and outof the holding structure 114.

The cleaning process in the supercritical fluid chamber may be furtherenhanced by the use of an agitation mechanism, which may applyultrasonic waves through the supercritical fluid, agitate the banknotes(or the structures that hold them), or otherwise agitate thesupercritical fluid.

In one embodiment, the banknotes have a thickness of 0.1 mm and can beheld in holders or trays separated from each other by a distance of 0.5mm. Based on this geometry, a supercritical fluid chamber having avolume on the order of 1 m³ can clean over 1 million notes per day.Given that the United States processes 30 billion banknotes each year,supercritical fluid chambers having a volume on the order of 100 m³would be able to clean all processed U.S. currency, even without sortingthe notes first.

To prevent the sebum that is stripped from the banknotes from coatingthe chamber or re-depositing on the banknotes, and to prevent thesupercritical CO₂ from saturating with the sebum that is in solution, atrapping material may be provided to remove the sebum from thesupercritical CO₂. While many trapping agents may be employed to stripthe sebum from the supercritical CO₂ solution, fumed silica ispreferably employed. The trapping material helps to prevent saturationof the supercritical fluid, and may be a high surface area material towhich the contaminants may attach. Fumed silica is a synthetic,amorphous, colloidal silicon dioxide. It is produced by the vaporhydrolysis of chlorosilanes, such as silicon tetrachloride, in ahydrogen-oxygen flame at 1800° C. In the combustion process, moltenspheres of amorphous silica are formed. Fumed silica is a white fluffypowder, consisting of spherically shaped primary particles, ranging inaverage from 7 to 40 nanometers in diameter, with a surface area of 400to 50 square meters per gram. Primary particles do not exist inisolation; they form aggregates and agglomerates. Technical propertiesof the fumed silica are not just determined by the primary particles,but also by the agglomerate size distribution. The fumed silica does nothave a clearly defined agglomerate size. The particle size distributionbecomes wider as the average primary particle size increases and thetendency to form agglomerates is reduced.

During the cleaning process, all of the CO₂ employed is preferablycaptured to prevent its release into the environment. The captured CO₂is further recycled for use in subsequent cleaning processes to reducethe overall environmental impact of the cleaning process. The cleaningprocess of the present invention minimizes the impact on the environmentby reducing the thousands of tons of shredded currency that must bedisposed of in a landfill or through burning.

FIG. 4 illustrates the cleaning cycle of the present invention. In analternate embodiment, a cleaning system may be provided in a cashstorage vault that is capable of supporting a supercritical fluid stateinside the vault to clean banknotes stored within it. This can beimplemented with banknotes that have yet to be processed, yielding ahigher yield of notes fit for recirculation after the standardprocessing by the central bank. Such a supercritical fluid cleaningchamber vault can also be implanted at commercial banks, which mayreceive a rebate for undertaking this step.

Testing was performed on banknotes using a high pressure supercriticalfluid chamber. An exemplary chamber is illustrated in FIG. 5, whichshows a $20 banknote inside it with CO₂ in the supercritical phase. Thechamber was made of ¾ aluminum with an observation window made of 1″plexiglass polymer. The chamber was constructed from cold drawn roundseamless mechanical tubing (MT-1018) with threaded top and bottom endcaps constructed of cold finished AISI C1018 steel bars. The assembledchamber had a diameter of 6.75″ and a length of approximately 12.75″.The diameter was 5.875″, leaving a wall thickness of 0.4375″. Thechamber had a dual ¼ (npt) threaded fittings machined into the cylinderwall before the cleaning chamber for filling an purging and a second setof ¼ (npt) threads in the top cover for installation of a pressuremonitor and a safety release valve. The fabricated components werecoated with 0.0001″-0.0003″ of electroless-nickel plating for corrosionresistance. The chamber could be operated at temperatures in the rangeof 25 C. and 60 C. and at pressures up to 2000 psi, at a duration of 30minutes to 12 hours. In addition, the chamber could be immersed in anaqueous ultrasonic bath to enhance the cleaning process.

The testing described herein was performed on all notes at the sametemperature and pressure. In short, the testing showed that sebum,coffee, and motor oil were removed from the banknotes withoutcompromising the notes' security features. Moreover, in one test, a U.S.$1 note having one colony of micrococcus luteus, a skin bacteria, and234 colonies of yeast (fungus) was cleaned and disinfected using themethod of the present invention, and none of the pathogens remained onthe note.

In testing the cleaning process of the present invention, banknotes werecoated with a sebum material primarily composed of 18% free fatty acids,37.8% beef tallow, and 18.3% lanoline. After being coated, the noteswere placed in a temperature controlled chamber for 8 days at 90° C. and65% relative humidity to simulate accelerated aging and circulation ofthe banknote. After oxidation takes place, the sebum developed ayellowish color, which along with the index matching effects, resultedin a soiled note resembling what is found in circulating currency. Forexample, FIG. 6 illustrates a side by side comparison of the same partof a new U.S. $1 banknote before and after coating and oxidation with asebum layer.

Once the notes were soiled, they were cleaned using supercritical CO₂ at50° C. and 1600 psi for 3 to 8 hours. Characterization was aimed atdetermining the survivability of various ambient light security featuresviewed under UV light, and machine readable features such as magneticand high level covert features such as ENIGMA (De La Rue International)and M (Gieseke and Devrient) before and after the cleaning process. Theremoval of sebum was studied by measuring the diffuse reflectancespectrum and UV features were characterized before and after using acalibrated fluorimeter. In addition, porosity was measured using aphotoporousimeter, developed in-house, which allowed for thedetermination of relative changes caused by the super critical CO₂cleaning process on U.S. banknotes. Pulp based banknotes from the U.S.,Europe, and China, as well as polymer banknotes made of biaxial orientedpolypropylene coated with an inorganic opacity layer prior to printing,were all tested using these methods.

Experiments were performed on a number of banknotes with a focus on U.S.banknotes made from paper which is approximately 75% cotton and 25%linen fibers and printed by the United States Bureau of Engraving andPrinting. Results of the cleaning process can be seen in FIG. 7. FIG. 7illustrates images of a U.S. $1 banknote before sebum treatment, aftersebum treatment, and after cleaning with supercritical CO₂ at 50 C. for8 hours with ultrasonic agitation. It is important to note that U.S.banknotes include a sebum-like dip which results in a yellowish coloringof banknotes, even when they are brand new. Accordingly, cleaning maynot look as efficient at removing all of the sebum as it really is,because of the limitation of the U.S. banknotes. Based on these results,the supercritical CO₂ cleaning process effectively removed oxidizedsebum from U.S. banknotes. The process removes on the order of 20% ofthe deposited sebum layer and appears to preferentially remove moietiesresponsible for absorption in the 500 nm to 650 nm region, which arelikely to be the larger fatty acid components of the mixture.

The results of the cleaning in this manner can also be seen in thegraphs of FIGS. 8A and 8B. Thus, it can be seen that the processdisclosed herein cleans a substantial amount of soil from the notes, asevidenced by the nearly 10% increase in the reflectance of the noteacross the near ultraviolet and visible spectrums. Such cleaning notonly enhances the cleanliness and appearance of the note, but alsoincreases the machine readability of the security features on the note.

As seen in FIG. 9, the overall results shown on a 5 Euro notedemonstrate clear results of the cleaning process. The left side of theimage shows the piece of the note which was cleaned using supercriticalCO₂ at 1600 psi and 55 C. for 8 hours. Before cleaning, the note wascoated with Bey sebum and stored at 90 C. and 70% relative humidity fornine days.

As another demonstration of the effectiveness of supercritical cleaningof banknotes, the process was tested on banknotes soiled with motor oil(e.g., Shell ASE 20). The data in FIG. 10 shows the diffuse reflectionspectra before and after cleaning.

The key to the viability for recycling of soiled banknotes using thesecleaning techniques is dry removal of the oxided oils and othercontaminants while maintaining the integrity and usefulness of theimportant and costly public and machine-readable security features ofthe banknotes. Optical studies of all the banknotes revealed that nochanges in the quality or contrast of the printing were observed aftercleaning, including the flexographic, gravure and intaglio and opticallyvariable inks.

Another feature of the present invention is that the security featureson the bank notes are either totally unaffected or weakly diminished bythe cleaning process. Notably, the magnetic inks, fluorescence of UVactive features, holograms, metalized and de-metalized threads, andoptically variable inks all remain intact and functioning after thecleaning process. As shown in FIG. 11, the fluorescence of the securitythread in a U.S. $20 note is wholly unaffected. In addition, thefluorescence of the thread in a U.S. $5 note is slightly reduced afterextreme exposure to supercritical CO₂; however, the performance is notdegraded so as to impair the visual and machine verification process.FIGS. 12A and 12B show the fluorescence of security threads in variousbanknotes, namely U.S. $5, U.S. $10, U.S. $20, U.S. $50, U.S. $100, and50 Ruppee notes, both before and after cleaning by exposure tosupercritical CO₂.

In addition to emissive security threads, polymeric security fibers suchas those typically found in many of the world's banknotes were examined.For example, the effects of the cleaning process on the fibers in theRussian Ruble were studied. The data shown in FIG. 13 illustrates therobustness of the UV excited emissive features to the cleaning processwith respect to the security fibers in the Russian 100 Ruble banknote.

Long UV excited emissive security features are also often printed on abanknote as well using lithographic, flexographic, gravure, and intagliomethods. Examples of this are the Yuan, the Euro, and the British Pound.Printed emissive features in these, as well as other currencies, werestudied, and results showed most of them to be highly robust asillustrated by the data for the Chinese Yuan in FIG. 14.

Experiments with the UK banknotes, which have a two color UV emissivepattern, revealed that these pigments were partially dissolved away.Experiments using only thermal exposure confirmed that this was eitherthe result of dissolution or reaction with the CO₂, and not the thermaldegradation of the fluorophore or phosphor. FIG. 15 illustrates thepattern before and after exposure to super critical CO₂ at 50 C. for 8hours and the spectral changes that occurred. It is clear from theresilience of the Chinese Yuan example that inks can be formulated to beresilient to the process of cleaning, but that some of the existing inkbases are not.

As previously discussed, machine-readable security features play animportant role in banknote security. The most common machine readablesecurity features are based on magnetic and capacitance and are mostoften utilized in single note acceptor applications from automatedteller machines to bill changers and vending machines.

The magnetic inks utilized in a number of banknotes, and particularlythe U.S. banknotes and European notes, were found to be robust andunchanged by the supercritical fluid cleaning process at 50 C. and forup to 16 hours. Capacitive machine readable features such as those usedin security threads, which rely on metallization, also survived testingup to 16 hours.

In addition to the machine-readable features, which are used in thepublic domain and by commercial banks, central banks employ one or morecovert features that are typically read at rates of up to 40banknotes/sec on high speed sorters. These features are only known tothe central banks, the enforcement authorities, and the companies thatsupply them. One of these technologies is the over thirty year oldM-feature, which was developed by Sigreid Otto of Geiseke and Devrient.This security feature proved to be resilient to the supercritical fluidcleaning process as it is based on an inorganic material. Like most ofthe emissive inks, the key to maintaining its robustness is in theproper choice of the base material if it is in a printed format. Thevarious Enigma security feature signatures from De La Rue Internationalwere tested and found to be robust and unchanged after the cleaningprocess for 16 hours at 50 C.

Another important parameter used to determine the fitness of banknotesis limpness. When banknotes have been in circulation, the mechanicalwear from folds, handling, and use in bill acceptors, results in a lossof mechanical elasticity that leads to the notes becoming limp. This“limpness” has been shown to be directly related to changes in theporosity of the banknote with mechanical wear. The porosity of thebanknotes increases with use and manifests itself in a lower effectiveelastic constant. Limpness is measured in automated sorting environmentsusing acoustics and ultrasonic reflection.

The porosity of banknotes was measured to determine the effects ofsupercritical CO₂ and elevated temperatures on the substrate.Supercritical CO₂ could cause swelling of the fiber network which couldhave a hysteresis and leave the banknotes more porous. It is alsopossible that since paper is a non-equilibrium network, that therelaxed, post-supercritical CO₂ treatment could be compacted relative tothe initial state.

The measurements were made using a home built transient gas diffusiondevice with Ar as the transport species. The Ar gas was opticallydetected on the other side of the note. The system utilized a solenoidvalve to create a burst of Ar, which was then detected as it diffusedthrough the network. In effect, the delay time was a measure of the voidfraction-totuousity product. FIGS. 16A and 16B illustrate the transientresponses for a U.S. $1 note, which is in circulation and one that hasnot been circulated, respectively. The figures demonstrate that theuncirculated note has lower porosity resulting in both a diminishedsignal and a longer delay relative to the pulse of Ar shown in theyellow trace. FIGS. 17A and 17B illustrate the signals of theuncirculated note before and after supercritical CO₂ cleaning and thatthe process has no effect on the porosity, and hence the limpness, ofthe note.

In addition to performing tests on individual banknotes, tests wereperformed on stacks of banknotes. Each of the stacks of banknotesincluded approximately 100 banknotes bound together with a strap (see,e.g., FIG. 18). Before and after cleaning of a stack of banknotes, thesoil level on the face and the back of each banknote in the stack wastested using a DLR banknote integrated diffuse reflectance soilingsensor. Generally, for any banknote, the diffuse reflectance valueinversely relates to the soil level of the banknote. For example, thehigher the diffuse reflectance value, the lower the soil level and thecleaner the banknote.

The method of cleaning banknotes disclosed herein may also be used toclean and restore other materials that may include images, painttextures, print, or combinations thereof without compromising theintegrity of the images, paint textures, and print. The materials may beones where restoration is desired including, but not limited todocuments and artwork, such as paintings. Like the method for cleaningthe banknotes, supercritical fluid, such as CO₂, may be used to removesubstances, including but not limited to, contaminants, dirt, sebum, andpathogens from the material without destroying any images, painttextures, or print that may be on the material.

The embodiments and examples above are illustrative, and many variationscan be introduced to them without departing from the spirit of thedisclosure or from the scope of the appended clams. For example,elements and/or features of different illustrative and exemplaryembodiments herein may be combined with each other and/or substitutedwith each other within the scope of this disclosure. The objects of theinvention, along with various features of novelty which characterize theinvention, are pointed out with particularity in the claims annexedhereto and forming a part of this disclosure. For a better understandingof the invention, its operating advantages and the specific objectsattained by its uses, reference should be had to the accompanyingdrawings and descriptive matter in which there is illustrated apreferred embodiment of the invention.

What is claimed is:
 1. An apparatus for cleaning a stack of banknotes,each banknote including a substrate, visual data and a security feature,comprising: a chamber containing a supercritical fluid at a temperatureand a pressure and for a duration sufficient to disinfect ordecontaminate the banknotes and not compromise the security feature andthe visual data of the banknotes; the chamber further containing astructure holding the stack of banknotes so that the supercritical fluidcirculates through and around the banknotes to remove one or moresubstances into the supercritical fluid; and a strapping machinesecuring a securing mechanism to the stack of banknotes; wherein thevisual data is printed on the substrate using Flexo inks, gravure inks,intaglio inks, dry color pigments blended with oils and extenders, orphosphor chips containing micro-interference layers; and wherein thesecurity feature includes laminated polymer or paper security threads,windowed security threads, quasi-public security threads, magnetic inksor threads, demetalized security threads, lenticular lens array securitythreads, fluorescent or non-fluorescent security threads, holographicfeatures, planchettes, digital watermarks, numeric watermarks, Guillochepatterns, microprinting, applied foils, blind embossings, latent imagesthat render different information from different viewing angles,optically variable pigments producing different color effects dependingon viewing angles, metallic effect inks, inks that fluoresce orphosphoresce under illumination with ultraviolet or infrared sources, orM features.
 2. The apparatus of claim 1, further comprising a sorter fordetermining whether the banknotes have one or more properties thatsatisfy one or more predetermined criteria.
 3. The apparatus of claim 2,wherein the sorter operates with the strapping machine.
 4. The apparatusof claim 1, wherein the securing mechanism comprises a band, a strap ora clip.
 5. The apparatus of claim 1, wherein the supercritical fluid inthe chamber is combined with an additive for removing marks on thebanknotes.
 6. The apparatus of claim 5, wherein the additive includes anoxalic acid.
 7. The apparatus of claim 1, wherein the supercriticalfluid in the chamber is combined with an additive for enhancing at leastone mechanical property of the banknotes.
 8. The apparatus of claim 7,wherein the additive includes at least one of an aqueous citric acidsolution and ammonium zirconium carbonate.
 9. The apparatus of claim 1,wherein the structure is a basket.
 10. The apparatus of claim 1, whereinthe stack of banknotes includes a material covering the stack ofbanknotes and having at least one slit allowing the supercritical fluidto circulate through and around the banknotes.
 11. The apparatus ofclaim 1, wherein to disinfect or contaminate includes to removedangerous chemicals from the banknotes.
 12. The apparatus of claim 11,wherein the dangerous chemicals include pesticides or mercury.
 13. Anapparatus for cleaning a stack of banknotes, each banknote including asubstrate, visual data and a security feature, comprising: a chambercontaining a supercritical fluid at a temperature and a pressure and fora duration sufficient to clean the banknotes and not compromise thesecurity feature and the visual data of the banknotes; and the chamberfurther containing a structure holding the stack of banknotes so thatthe supercritical fluid circulates through and around the banknotes toremove one or more substances into the supercritical fluid; wherein thevisual data is printed on the substrate using Flexo inks, gravure inks,intaglio inks, dry color pigments blended with oils and extenders, orphosphor chips containing micro-interference layers; and wherein thesecurity feature includes laminated polymer or paper security threads,windowed security threads, quasi-public security threads, magnetic inksor threads, demetalized security threads, lenticular lens array securitythreads, fluorescent or non-fluorescent security threads, holographicfeatures, planchettes, digital watermarks, numeric watermarks, Guillochepatterns, microprinting, applied foils, blind embossings, latent imagesthat render different information from different viewing angles,optically variable pigments producing different color effects dependingon viewing angles, metallic effect inks, inks that fluoresce orphosphoresce under illumination with ultraviolet or infrared sources, orM features.
 14. The apparatus of claim 13, wherein the supercriticalfluid contains at least one additive.
 15. The apparatus of claim 14,wherein the additive is for removing marks on the banknotes.
 16. Theapparatus of claim 15, wherein the additive includes an oxalic acid. 17.The apparatus of claim 14, wherein the additive is for enhancing atleast one mechanical property of the banknotes.
 18. The apparatus ofclaim 17, wherein the additive includes at least one of an aqueouscitric acid solution and ammonium zirconium carbonate.
 19. The apparatusof claim 13, wherein to clean the banknotes includes removing pathogensfrom the banknotes.
 20. The apparatus of claim 19, wherein pathogensincludes bacteria and viruses.